Fibre Bragg-grating sensor

ABSTRACT

The subject matter of the present invention is a fiber Bragg grating sensor  1, 25  which is suitable, in particular, for measuring differential pressures and flow rates v 1  in oil drill holes. The sensor principle according to the invention is based on using a transducer  1  with two pressure chambers  7   a ,  7   b  to convert a hydrostatic pressure difference between two liquid or gaseous media  11   a   , 11   b  into a longitudinal fiber elongation or fiber compression and measuring it via the displacement of the Bragg wavelength Δλ B  of at least one fiber Bragg grating  3, 4 . Exemplary embodiments are specified which have two fiber Bragg gratings  3, 4  which are sensitive to elongation in opposite senses and which have temperature-compensating transducers  1 , and which have a plurality of transducers  1  in a wavelength-division-multiplexing configuration. One embodiment relates to measuring a flow rate v 1  with the aid of a venturi tube  23.

The present invention relates to the field of fiber-optic pressure andtemperature measurement. It proceeds from a fiber-optic sensor accordingto the preamble of claims 1 and 12.

In oil production, drill holes have to be monitored with regard topressure and temperature. The liquid pressures in the drill hole can beup to 100 MPa (1000 bar), and the temperatures can be up to over 200° C.Electric sensors such as, for example, piezoelectric resistors,piezoelectric elements, capacitive probes or crystal resonators, oroptical pressure sensors such as, for example, Fabry-Perot resonators orelastooptic sensors are frequently used in pressure measurement up toapproximately 170° C.

A fiber-optic pressure sensor in accordance with the preamble is knownfrom the article by M. G. Xu et al., “Optical In-Fibre Grating HighPressure Sensor”, Electronics Letters 29 (4), pages 398-399 (1993).There, fiber Bragg grating sensors are presented for measuring isotropicpressures of liquids. The Bragg grating of a sensor fiber is exposeddirectly to the all round hydrostatic pressure of a fluid. A substantialdisadvantage consists in that the isotropic pressure sensitivity forBragg gratings in glass fibers is exceptionally low (typically 0.0003nm/100 kPa specific Bragg wavelength displacement at 1550 nm). Inaddition, because of the high temperature sensitivity (typically 0.01nm/° C.), it is necessary to compensate temperature effects.

An optical sensor with fiber Bragg gratings for measuring materialelongations is disclosed, for example, in U.S. Pat. No. 4,761,073. Forthe purpose of monitoring body deformations, the sensor fiber istypically fastened on the surface of the body or embedded in the body.It is proposed to eliminate signal interference owing to thermal gratingelongations with the aid of superimposed gratings of various reflectionwavelengths.

U.S. Pat. No. 5,042,898 exhibits a temperature-stabilized fiber Bragggrating which can be used as wavelength standard to stabilize theemission wavelength of laser diodes, or as a wavelength filter in fiberoptic sensors. The fiber is held between two supports of suitablethermal expansion and length such that the thermally induced changes inthe Bragg wavelength are compensated.

It is the object of the present invention to specify a fiber Bragggrating pressure sensor which is suitable for measuring differentialisotropic pressures in liquids or gases and is distinguished by goodmeasuring sensitivity and a large measuring range. This object isachieved according to the invention by means of the features of claims 1and 12.

The invention specifies a fiber-optic sensor for differential pressuremeasurements which comprises a transducer with pressure members forholding two fluids, the transducer being configured for converting themedium pressures into a longitudinal elongation or compression of atleast one fiber Bragg grating of a sensor fiber. The transducertherefore exchanges pressure with the two fluids, is deformed by theirpressures and transforms the deformation into a change in length of thesensor fiber in the region of a fiber Bragg grating. The deformation ofthe transducer depends on the absolute pressures and/or directly on thedifferential pressure.

In first exemplary embodiments, a fiber Bragg grating is held betweentwo pressure members which can be elongated by the pressures of thefluids.

In second exemplary embodiments, a fiber Bragg grating is held between asupporting member fastened on the transducer housing and a pressuremember which can be elongated by the pressure difference between the twofluids.

In addition, for the purpose of error compensation, a fiber Bragggrating can be fitted between the pressure members or a pressure memberand supporting member such that the measuring signal is oppositelydirected and interfering signals are codirectional, and a doublednoise-free difference signal can be formed.

Another exemplary embodiment constitutes a serial, reflexive multiplexarrangement of a plurality of fiber Bragg grating differential pressuresensors with different Bragg wavelengths which are fed via a commonbroadband light source and detected in a wavelength-selective fashion.

A preferred application of the differential pressure sensor is use inconjunction with a venturi tube for the purpose of determining a flowrate.

Further designs, advantages and applications of the invention followfrom the dependent claims and from the description, which now follows,with the aid of the figures.

With reference to a differential pressure sensor according to theinvention, in the drawing:

FIG. 1 shows a transducer (=pressure transmission element) with twoconcentric pressure cylinders: (a) arrangement for the elongation of afiber Bragg grating; (b) arrangement with temperature-compensatingpressure cylinders; (c) arrangement for an oppositely directedelongation of two fiber Bragg gratings for compensating signalinterference from temperature and all round pressure of a medium;

FIG. 2 shows a transducer with two serial pressure cylinders (a) for theelongation of a fiber Bragg grating, or (b) for the oppositely directedelongation of two fiber Bragg gratings;

FIGS. 3(a), (b) show a transducer with two parallel pressure cylindersfor the elongation of a fiber Bragg grating;

FIG. 4 shows a transducer with two pressure cylinders for a separateelongation of two fiber Bragg gratings for the purpose of measuring twoabsolute pressures;

FIG. 5 shows a multiplex arrangement with a plurality of differentialpressure sensors in reflection; and

FIG. 6 shows a venturi tube with differential pressure sensor for thepurpose of determining flow rates.

Identical parts are provided with identical reference symbols in thefigures.

The subject matter of the invention is a fiber-optic pressure sensor.The known measuring principle consists in that a fiber Bragg gratingwhich is written into a monomode fiber by UV light acts as a reflectionor transmission filter with a characteristic Bragg wavelength λ_(B).Longitudinal fiber elongations change the grating period and refractiveindex and displace the Brag wavelength λ_(B). The output signals arewavelength-coded and independent of the light power. The measuring rangeis limited only by the fiber ultimate strength in the case of elongationmeasurements with the aid of Bragg gratings.

The invention is explained firstly with regard to FIGS. 1-4. Thefiber-optic pressure sensor 1, 25 comprises a transducer 1 with a sensorfiber 2 which has at least one fiber Bragg grating 3, 4, 5, comprisingat least one first pressure member 7 a for holding a first medium 11 aunder an all round pressure p₁, comprising at least one second pressuremember 7 b for holding a second medium 11 b under an all round pressurep₂, and being configured for measuring a pressure difference p₁-p₂ byconverting the all round pressures p₁, p₂ into a longitudinal elongationor compression of at least one fiber Bragg grating 3, 4 of the sensorfiber 3. The transducer is advantageously configured for a differentialelongation of the fiber Bragg grating 3, 4 induced by the pressuredifference p₁-p₂. In particular, the sensor 1, 25 is suitable formeasuring differential pressures and flow rates in oil drill holes.

In the exemplary embodiments illustrated, the sensor fiber 2 is mountedbetween holders 6 a, 6 b, 6 c; 15 b and preferably prestressed, theholders 6 a, 6 b, 6 c; 15 b are connected in a force-closed fashion tothe pressure members 7 a, 7 b and, if appropriate, to supporting members15 a, and the pressure members 7 a, 7 b are configured to deflect atleast one holder 6 a, 6 b, 6 c as a function of the pressures p₁, p₂,Preferably, exactly two cylindrical pressure members 7 a, 7 b areprovided, which are arranged concentrically, in parallel or seriallyrelative to one another, the pressure cylinders 7 a, 7 b have the samelength L and the holders 6 a, 6 b, 6 c are fastened on plunger faces 8,8 a, 8 b of the pressure cylinders 7 a, 7 b.

The transducer 1 is to have separate inlets 10 a, 10 b for the media 11a 11 b into the pressure members 7 a, 7 b. A fiber Bragg grating 3 canbe provided for differential pressure measurement, a fiber Bragg grating4 can be provided for error compensation, and/or a fiber Bragg grating 5can be provided for temperature measurement. Typically, of the fiberBragg gratings, 3 is always, 4 is sometimes and 5 is not mechanicallyprestressed. They are characterized by different Bragg wavelengths λ_(B)and can be read out spectrally in a separate fashion.

The transducer 1 has pressure-tight fiber bushings 12 a, 12 b for thesensor fiber 2 and/or a cavity 13 for a fiber Bragg grating 5 for thepurpose of temperature measurement. At least one block with a bore forlateral support of the sensor fiber 2 in the region of a fiber Bragggrating 3, 4 is to be provided for a compression arrangement (notillustrated). A very much larger pressure measuring range can berealized because glass fibers can be loaded 20 times more in terms ofpressure than elongation.

FIGS. 1 and 3 show arrangements in which a fiber Bragg grating 3 isfixed for the purpose of differential pressure measurement by holders 6a, 6 b between the first and second pressure member 7 a, 7 b. Inparticular, for the purpose of antiphasal change in elongation, inaccordance with FIG. 1c an error compensation fiber Bragg grating 4 canbe fastened, between holders 6 a, 6 c, in reverse sequence between thesecond and first pressure members 7 b, 7 a. That is to say, the sensorfiber sections with the fiber Bragg gratings 3, 4 are arranged on bothsides of the end plate or plunger face 8 of the first pressure cylinder7 a and are connected at their opposite ends to the second pressurecylinder 7 b. As a result, elongations owing to differential pressuresp₁-p₂ are opposed to one another, and interfering elongations owing toisotropic pressure, temperature dependencies of the fiber Bragg gratings3, 4 and thermal expansion of the pressure members 7 a, 7 b are renderedcodirectional. It is therefore possible to eliminate the interferencesignals and double the useful signal by forming a difference signalbetween the first and second fiber Bragg grating 3, 4.

FIG. 2 show arrangements in which a fiber Bragg grating 3 is mounted, onholders 6 a, 15 b, between a holder 6 a, which can be deflected bydifferential pressure between two pressure members 7 a, 7 b, and asupporting member 15 a, which is permanently connected to the transducerhousing 9. The pressure members 7 a, 7 b are preferably arrangedserially one behind another and have a common end plate 8 by which theholder 6 a is connected. In particular, in FIG. 2b a prestressed errorcompensation fiber Bragg grating 4 is held (6 a, 15 b) for the purposeof antiphasal change in elongation in reverse sequence between thesupporting members 15 a and the holder 6 a which can be deflected bydifferential pressure. That is to say, the fiber Bragg gratings 3 and 4are connected on both sides of the holder 6 a to the substantially fixedsupporting member 15 a via the holders 15 b. The above discussedcompensation according to the invention of interference effects in thedifferential signal can be achieved, in turn, thereby.

A detailed analysis of the mode of operation of the differentialpressure sensor 1 is given with the aid of FIG. 1a. The first pressurecylinder 7 a is mounted on a projection or base 14, is sealed at theother end by an end plate 8 a and subjected to an internal pressure p₁and an external pressure p₂, The concentric second pressure member 7 bis mounted on the housing 9, has an open end plate 8 b and is exposed tothe second pressure p₂ inside and outside via the inlet 10 b. L denotesthe length of the pressure cylinder 7 a, 7 b, and l denotes the lengthof the elongation length of the sensor fiber 2 and the length of thebase 14. A variant with parallel pressure members 7 a, 7 b is shown inFIG. 3b.

The differential longitudinal elongation L of the pressure members 7 a,7 b depends on the pressure-induced longitudinal stresses and also, viathe Poisson transverse elongation, on the radial and tangential stressesin two pressure members 7 a, 7 b. The result for pressure members 7 a, 7b of equal length L, equal modulus of elasticity E and equal Poissonnumber μ is

L=Lξp/Eξ(1-2μ)ξR _(i) ²/(R _(a) ² −Ri ²),  (G1)

R_(i) being the inside radius and R_(a) being the outside radius of theclosed pressure member 7 a loaded by the differential pressure p=p₁-p₂.The differential elongation L does not depend on the absolute pressuresp₁, p₂ or on the radii of the pressure members 7 b. L is transferredonto the fiber elongation distance l and effects a wavelengthdisplacement of

O _(B)=1.21 μm ξL/l  (G2)

for a fiber Bragg grating 3, 4 with a Bragg wavelength O_(B) at 1550 nm.On the fiber elongation distance, the prestressing is to be dimensionedsuch that it does not vanish even in the case of maximum pressureloading. Owing to the length ratio L/l, the magnitude of the fiberelongation can be prescribed for a given transducer elongation and can,in particular, be selected as large for a high pressure resolution. Forexample, a length ratio of L/l>10 for the purpose of mutual tuning ofthe linear, hysteresis-free regions of the transducer elongation(ΔL/L<0.001) and fiber elongation (Δl/l up to over 0.01).

A quantitative example of achievable resolution and measuring range ofthe differential pressure: pressure members 7 a, 7 b made from steelwith E=196·10⁹ N/m², M=0.28, L=150 mm, l=10 mm, R_(i)=4.8 mm, R_(a)=5.0mm. The specific displacement of the Bragg wavelengths is then Δλ/Δp=480pm/MPa, and the pressure resolution is 2.1 kPa for 1 pm wavelengthresolution. The measuring range is bounded by the elastic limit of thetransducer 1 to differential pressures of up to approximately 5 MPa(Bragg wavelength displacement Δλ_(B)=2.4 nm). The radii of the secondpressure member 7 b are non-critical and can be 6 mm and 8 mm, forexample. A transducer housing 9 with an inside radius of 7.5 mm and anoutside radius of 10.5 mm can withstand absolute pressures above 100MPa.

The Bragg wavelength λ_(B) of the fiber Bragg grating 3 can also bedisturbed directly by the isotropic pressure p₂ (Δλ_(B)=a few pm/MPa),inherent thermal elongation (10.3 pm/° C. at λ_(B)=1550 nm) ordifferential thermal elongation of the pressure members 7 a, 7 b. Inaccordance with FIG. 1c, for compensation purposes a second fiber Bragggrating 4, which can be read out spectrally in a separate fashion, isexposed on an elongation distance l of the same length to the samepressure p₂, the same temperature and the same thermal elongation, andthe noise-free difference signal of the two fiber Bragg gratings 3, 4 isevaluated. Moreover, the temperature of the transducer 1 can bemonitored, by means of a third, mechanically unloaded fiber Bragggrating 5 and, if appropriate, be used to correct a differentialpressure signal.

In accordance with FIG. 1b, it is possible to provide passivetemperature compensation for the fiber elongation distance(s) as analternative or in addition. For this purpose, at least one pressuremember 7 a, 7 b and/or at least one supporting member 15 a is to consistof or be assembled from materials with different coefficients of thermalexpansion α₁, α₂, such that a differential thermal expansion between theholders 6 a, 6 b, 6 c counteracts a thermally induced displacement of aBragg wavelength λ_(B) of the sensor fiber 2. It holds in the case ofcomplete temperature compensation that

(α₂ ·L−α₁·(L+l))/l=8.0·10⁻⁶° C.⁻¹  (G3)

α₁, α₂ being the coefficient of thermal expansion of the first pressuremember 7 a (including the base 14), and of the second pressure member 7b. By contrast with the U.S. Pat. No. 5,042,898 mentioned at thebeginning, according to the invention equation G3 is used to select thecylinder length straight away, and the expansion coefficients arematched. Assuming that L=150 mm, l=10 mm and α₁=12.4·10⁻⁶° C.⁻¹, it isnecessary for α₂=14.0·10⁻⁶° C.⁻¹. Moreover, the fiber prestressing is tobe selected high enough to ensure adequate prestressing even in the caseof maximum operating temperature and maximum pressure difference p₂-p₁.The reliability of the differential pressure measurement is clearlyimproved by the temperature compensation.

In addition to linear coefficients of thermal expansion in accordancewith equation G3, suitable transducer materials are also to have a lowdegree of nonlinearity in thermal expansion, a high corrosion resistanceof up to 230° C., a similar modulus of elasticity E and a similarPoisson number μ. This restricts the selection of steels, and in manyinstances passive temperature compensation cannot be carried out, or canbe carried out only incompletely. According to the invention, thepressure or supporting members 7 a, 7 b, 15 a can be assembled from atleast two segments with different coefficients of thermal expansion andprescribable lengths L′, L″. In the exemplary embodiment according toFIG. 1b, the second cylinder 7 b is constructed from segments L″ with α₁and L′ with α₂. The modified condition for the temperature compensationruns

(α₂ ·L′−α₁·(L′+l))/l=8.0·10⁻⁶° C.⁻¹  (G4)

Thus, for given coefficients of expansion α₁, α₂, the differentialexpansion of the pressure members 7 a, 7 b can be tailored by selectingthe segment lengths L′, L″ (where L′+L″=L). For example, a nickel-basedalloy (for example “Hastealloy C-22” from Hynes International withα₁=12.4·10⁻⁶° C.⁻¹) is combined with a chromium-nickel steel (forexample “AISI 304” with α₂=17.0·10⁻⁶° C.⁻¹). For L=150 mm and l=15 mmthe result is L′=44.3 mm and L″=105.6 mm.

An advantage of the temperature-compensated arrangement according toFIG. 1b consists in that only the first fiber Bragg grating 3 ismechanically prestressed. Interference owing to isotropic pressure p₂ isdetected with the aid of the now unloaded fiber Bragg grating 4 and thetemperature dependence of the latter is corrected with the aid of thefiber Bragg grating 5. The passive temperature compensation inaccordance with FIG. 1b reduces the Bragg wavelength spectral regionrequired for a fiber sensor 1. It can be applied in principle in thecase of all exemplary embodiments.

The arrangements according to FIGS. 2a, 2 b and 3 a have the advantagethat the fiber Bragg gratings 3, 4, 5 are not exposed to the pressure ofthe medium 11 b. The interior of the transducer 1 outside the pressuremembers 7 a, 7 b can be filled with a vacuum or a low-pressure gas. Thepressure members 7 a, 7 b are to be designed for the full pressureloading p₁ or p₂, The measuring range for differential pressures thenextends up to p₁ or p₂. The pressure resolution is approximately 100 kPafor L=150 mm, wall thicknesses designed up to 100 MPa and a 1 pmspectral resolution. FIG. 3a shows a variant with two parallel pressuremembers 7 a, 7 b, which are loaded exclusively by internal pressure p₁or p₂, a prestressed fiber Bragg grating 3 for differential pressuremeasurement, and an unloaded fiber Bragg grating 5 for temperaturemeasurement. The fiber Bragg grating 3 is held 6 a, 6 b betweencylinders 7 a, 7 b of equal length via an end place 8 a and an end plate8 b lengthened by the base 14 of length l.

FIG. 4 shows a further differential pressure sensor 1, in the case ofwhich one fiber Bragg grating 3 each is held 6 a, 15 b between a firstpressure member 7 a and a supporting member 15 a, and between a secondpressure member 7 a and a supporting member 15 a, and a pressuredifference p=p₁-p₂ can be determined from the separately measuredelongations of the fiber Bragg gratings 3, 4. The compact arrangement oftwo absolute pressure measurements in a transducer 1 is advantageous inthis case.

FIG. 5 shows a multiplex arrangement 25 with a plurality of transducers1, according to the invention, of different Bragg wavelengths O_(B). Thetransducers 1 are optically connected to a broadband light source 16,for example an LED or SLD and, preferably via a fiber coupler 18, to awavelength-division demultiplexer 19 and a detector plus an electronicmeasuring system 20 (and computer 21). 22 denotes an optional source ofreference wavelengths for spectral calibration of the fiber Bragggratings 3, 4, 5. The gratings have a spectral width of approximately0.2 nm, a maximum reflectivity of 90%, a length of 10 mm and tuningranges of 2.4 nm for temperature (0° C.-230° C.) and 3.6 nm fordifferential pressure measurement (0.003 maximum elongation). With a 1nm standby spacing relative to the tuning range of the adjacent grating,a passive temperature-compensated transducer 1 therefore requires a 7 nmspectral width. 7 transducers 1 can be multiplexed by wavelength with alow loss 1550 nm light source (50 nm spectral width). Alternatively, orin addition, the transducers 1 can also be read out sequentially oneafter another using a time-division multiplexing method and/or by meansof fiber-optic switches.

FIG. 6 shows one use of a fiber-optic differential pressure sensor 1, 25according to the invention, in the case of which a flow rate v₁ of afluid flow 24 is determined from a differential pressure measurement. Inparticular, the inlets 10 a, 10 b of the transducer 1 are connected to aventuri tube 23 at two locations with cross-sectional areas A₁ and A₂.The flow rate v₁ can be determined in a known way from the differentialpressure Δp=p₁-p₂.

The fiber-optic pressure sensor 1, 25 is characterized overall by anadvantageous interaction between transducer 1, which can be exposed toextreme pressure loads, and the fiber Bragg grating 3, 4, which is verysensitive to elongation, of the sensor fiber 2. It is possible as aresult to measure differential pressures of between 0.1 kPa and 10 MPaat very high absolute pressures of up to approximately 100 MPa with highresolution. A further advantage consists in that the pressure signal iswavelength-coded, and thus very insensitive to interference. It can beread out directly using fiber optics over large distances between thepassive sensor head 1 and the optoelectronic measuring device 16, 19-22.Also advantageous are the good high-temperature capability, corrosionresistance and insensitivity to electromagnetic interference. Because ofits compactness, the sensor 1, 25 is particularly suitable for measuringdifferential pressures and flow rates in drill holes.

List of Reference Symbols

1 Fiber optic differential pressure sensor (transducer) 2 Optical fiber,sensor fiber 3 Fiber Bragg grating 1 (for pressure measurement) 4 FiberBragg grating 2 (for compensation measurement) 5 Fiber Bragg grating 3(for temperature measurement) 6a-6c,15b Holders, fiber holders, ferruleholders 7a-7b Pressure members, pressure cylinders 7a Pressure cylinder1 (internal pressure p_(i)) 7b Pressure cylinder 2 (reference pressurep₂), reference cylinder 8, 8a, 8b End plates of the pressure members,plunger face 9 Transducer housing 10a, 10b Inlets 11a Medium 1, fluid 1(under pressure p₁) 11b Medium 2, fluid 2 (under pressure p₂) 12a, 12bPressure-tight fiber bushings 13 Cavity for temperature sensor fiber 14Projection, base 15a Supporting member, supporting cylinder 16(Broadband) light source, LED, SLD 17 Feeder fibers 18 Coupler, fibercoupler 19 Wavelength-division demultiplexer, tunable spectral filter,Fabry-Perot filter 20 Detector and electronic measuring system 21Computer, PC 22 Source for reference wavelengths 23 Venturi tube 24Fluid flow 25 Overall sensor A₁, A₂ Cross-sectional surfaces α₁, α₂Coefficients of thermal expansion Ε Young's modulus of elasticity lLength of the elongation distance of the pressure sensor fiber L Lengthof a pressure cylinder ΔL Differential elongation L′, L″ Segment lengthsof a pressure cylinder/supporting cylinder λ_(B) Bragg wavelength Δλ_(B)Bragg wavelength displacement μ Poisson number p₁, p₂ Pressures ΔpPressure difference R_(i) Inside radius of the first pressure cylinderR_(a) Outside radius of the first pressure cylinder v₁, v₂ Flow rates

What is claimed is:
 1. A fiber-optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure P₂, and the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ into a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber.
 2. Thefiber optic pressure sensor as claimed in claim 1, wherein thetransducer is configured for a differential elongation of the fiberBragg grating induced by the pressure difference p₁-p₂.
 3. The fiberoptic sensor as claimed in claim 1, wherein the sensor fiber is mountedbetween holders, the holders are connected in a force-closed fashion tothe pressure members and to supporting members, and the pressure membersare configured to deflect at least one holder as a function of thepressures p₁, p₂.
 4. A fiber optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure p₂, the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ into a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber, the sensorfiber is mounted between holders, the holders are connected in aforce-closed fashion to the pressure members and, optionally, tosupporting members, the pressure members are configured to deflect atleast one holder as a function of the pressures p₁, p₂, exactly twocylindrical pressure members are provided, which are arrangedconcentrically, in parallel or serially relative to one another, thepressure cylinders have the same length L, and the holders are fastenedon plunger faces of the pressure cylinders.
 5. The fiber optic sensor asclaimed in claim 1, wherein the transducer has separate inlets for themedia into the pressure members and/or a fiber Bragg grating is providedfor differential pressure measurement, a fiber Bragg grating is providedfor error compensation, and/or a fiber Bragg grating is provided fortemperature measurement.
 6. A fiber optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure p₂, the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ into a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber, and a fiberBragg grating is held between the first and second pressure members forthe purpose of differential pressure measurement.
 7. A fiber opticpressure sensor comprising a transducer with a sensor fiber which has atleast one fiber Bragg grating, wherein the transducer comprises at leastone first pressure member for holding a first medium under an all roundpressure p₁, the transducer comprises at least one second pressuremember for holding a second medium under an all round pressure p₂, thetransducer is configured for measuring a pressure difference p₁-p₂ byconverting the all round pressures p₁, p₂ into a longitudinal elongationor compression of the at least one fiber Bragg grating of the sensorfiber, and a fiber Bragg grating is held between a holder, which can bedeflected by differential pressure of two pressure members, and asupporting member.
 8. A fiber optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure p₂, the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ in a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber, one fiberBragg grating each is held between a first pressure member and asupporting member and a second pressure member and a second supportingmember, and a pressure difference can be determined with the aid of thetwo fiber Bragg gratings.
 9. A fiber optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure p₂, the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ into a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber, and atleast one pressure member and/or at least one supporting member consistsof, or is assembled from materials with different coefficients ofthermal expansion α₁, α₂, such that a differential thermal expansionbetween the holders counteracts a thermally induced displacement of aBragg wavelength of the sensor fiber.
 10. The fiber optic pressuresensor as claimed in claim 1, wherein the transducer has pressure-tightfiber bushings for the sensor fiber, and/or the transducer has a cavityfor a fiber Bragg grating for the purpose of temperature measurement,and/or at least one block with a bore for laterally supporting thesensor fiber is provided in the region of a fiber Bragg grating for thepurpose of a compression arrangement.
 11. The fiber optic pressuresensor as claimed in claim 1, wherein a plurality of transducers ofdifferent Bragg wavelength λ_(B) are optically connected to a broadbandlight source.
 12. Use of a fiber optic differential pressure sensorcomprising a transducer with a sensor fiber which has at least one fiberBragg grating, wherein (he transducer comprises at least one firstpressure member for holding a first medium under an all round pressurep₁, the transducer comprises at least one second pressure member forholding a second medium under an all round pressure p₂, and thetransducer is configured for measuring a pressure difference p₁-p₂ byconverting the all round pressures p₁, p₂ into a longitudinal elongationor compression of the at least one fiber Bragg grating of the sensorfiber, wherein a flow rate v₁ of a fluid flow is determined from adifferential pressure measurement.
 13. Use of a fiber optic differentialpressure sensor as claimed in claim 12, wherein the differentialpressure measurement is carried out at a venturi tube.
 14. The fiberoptic pressure sensor as claimed in claim 3, wherein the sensor fiber isprestressed.
 15. The fiber optic pressure sensor as claimed in claim 6,wherein an error compensation fiber Bragg grating is held between thesecond and first pressure members in reverse sequence for the purpose ofantiphasal change in elongation.
 16. The fiber optic pressure sensor asclaimed in claim 7, wherein the holder is connected to a common endplate of two serially arranged pressure members.
 17. The fiber opticpressure sensor as claimed in claim 7, wherein an error compensationfiber Bragg grating is held between the supporting member and the holderwhich can be deflected by differential pressure in reverse sequence forthe purpose of antiphasal change in elongation.
 18. The fiber opticpressure sensor as claimed in claim 9, wherein a pressure or supportingmember is assembled from at least two segments with differentcoefficients of thermal expansion and prescribable lengths L′, L″. 19.The fiber optic pressure sensor as claimed in claim 1, wherein aplurality of transducers of different Bragg wavelength λ_(B) areoptically connected to a broadband light source and via a fiber couplerto a wavelength-division demultiplexer and a detector plus an electronicmeasuring system.
 20. A fiber optic pressure sensor comprising atransducer with a sensor fiber which has at least one fiber Bragggrating, wherein the transducer comprises at least one first pressuremember for holding a first medium under an all round pressure p₁, thetransducer comprises at least one second pressure member for holding asecond medium under an all round pressure p₂, the transducer isconfigured for measuring a pressure difference p₁-p₂ by converting theall round pressures p₁, p₂ into a longitudinal elongation or compressionof the at least one fiber Bragg grating of the sensor fiber, and a fiberBragg grating is provided for differential pressure measurement, and afiber Bragg grating is provided for error compensation.
 21. A fiberoptic pressure sensor comprising a transducer with a sensor fiber whichhas at least one fiber Bragg grating, wherein the transducer comprisesat least one first pressure member for holding a first medium under anall round pressure p₁, the transducer comprises at least one secondpressure member for holding a second medium under an all round pressurep₂, the transducer is configured for measuring a pressure differencep₁-p₂ by converting the all round pressures p₁, p₂ into a longitudinalelongation or compression of the at least one fiber Bragg grating of thesensor fiber, and at least one block with a bore for laterallysupporting the sensor fiber is provided in the region of a fiber Bragggrating for the purpose of a compression arrangement.